The present invention relates to the viewing of holograms. A particular application is the viewing of holograms in non-coherent light.
A proposal for white light display of a hologram is to be found in an article "Display of Holograms in White Light", C. B. Burckhardt, Bell Systems Technical Journal, December 1966. pages 1841-4. A hologram is illuminated from behind. In front of the hologram is a venetian blind and a plane grating whose pitch corresponds to an interference pattern formed by light beams at the same angle as the mean angle between the subject beam and the reference beam used in making the hologram. Thus the average fringe spacial frequency of the hologram is equal to that of the grating. The slats of the venetian blind are at such an angle that they block direct light but allow through the first order diffracted light. This will be subject to severe colour dispersion but the second diffraction by the plane grating compensates for this so that the viewer sees a monochrome three-dimensional image behind the hologram, provided that the viewing and illuminating angles are correct.
In spite of this proposal in 1966 and increasing interest in holograms practical white light holograms displaying full parallax have not become readily available. Nor has it been possible to provide simple, direct viewing multi-colour holograms.